Total synthesis of bryostatins: the development of methodology for the atom-economic and stereoselective synthesis of the ring C subunit

Bryostatins, a family of structurally complicated macrolides, exhibit an exceptional range of biological activities. The limited availability and structural complexity of these molecules makes development of an efficient total synthesis particularly important. This article describes our initial efforts towards the total synthesis of bryostatins, in which chemoselective and atom-economical methods for the stereoselective assembly of the ring C subunit were developed. A Pd-catalyzed tandem alkyne-alkyne coupling/6-endo-dig cyclization sequence was explored and successfully pursued in the synthesis of a dihydropyran ring system. Elaboration of this methodology ultimately led to a concise synthesis of the ring C subunit of bryostatins.     

A unified strategy for the asymmetric total syntheses of diversonol and lachnone C

A unified synthetic strategy for the asymmetric syntheses of the natural products diversonol and lachnone C was developed by using the domino vinylogous aldol-oxa-Michael reaction as the enantioselective key step. Further transformations include dihydroxylation, lactol-opening by a Wittig-reaction, and lactonization. The obtained chromone lactones, a class of mycotoxins, can further be converted to tetrahydroxanthones by a Dieckmann condensation. This general method allows for the first time the enantioselective access to these classes of natural products and should be applicable to other members of the tetrahydroxanthone and chromone lactone families.     

Total syntheses of zaragozic acids A and C by a carbonyl ylide cycloaddition strategy

A carbonyl ylide cycloaddition approach to the squalene synthase inhibitors zaragozic acids A and C is described. The carbonyl ylide precursor 8 was synthesized starting from di-tert-butyl D-tartrate (47) via an eleven-step sequence involving the regioselective reduction of the mono-MPM (MPM=4-methoxybenzyl) ether 48 with LiBH4 and the diastereoselective addition of sodium tert-butyl diazoacetate to alpha-keto ester 10. The reaction of alpha-diazo ester 8 with 3-butyn-2-one (40) in the presence of a catalytic amount of [Rh2(OAc)4] gave the desired cycloadduct 59 as a single diastereomer. The dihydroxylation of enone 59 followed by sequential transformations permitted the construction of the fully functionalized 2,8-dioxabicyclo[3.2.1]octane core 5. Alkene 79 derived from 5 serves as a common precursor to zaragozic acids A (1) and C (2), since the elongation of the C1 alkyl side chain can be attained by olefin cross-metathesis, especially under the influence of Blechert's catalyst (85).     

Studies towards the total synthesis of (−)-borrelidin: a strategy for the construction of the C11-C15 cyanodiene fragment and the utility of RCM for macrocyclization using model systems

We report here a methodology for the construction of a conjugated cyanodiene synthon and the propensity of such synthons to participate in the olefin metathesis reaction. To this end, we have developed a strategy for the construction of the C11-C15 fragment of borrelidin and demonstrated the utility of the RCM reaction in the preparation of the final macrolide. To our knowledge, this is the first example of a RCM with a nitrile functionality on a diene.     

Studies on the synthesis of apoptolidin: synthesis of a C1-C27 fragment of apoptolidin D

Synthesis of a C(1)-C(27) fragment, a key intermediate in the synthesis of apoptolidin D, is reported. The synthesis involves a combination of Heck coupling and Horner-Wadsworth-Emmons reaction for the C(1)-C(7) trienoate portion and an efficient Suzuki cross-coupling protocol for the C(10)-C(13) diene portion.     

Expedient access to the okadaic acid architecture: a novel synthesis of the C1-C27 domain.

A newly designed synthetic entry to the C1-C27 domain of okadaic acid has been developed. This incorporates substantial improvements in the preparations of the key okadaic acid building blocks representing the C3-C8, C9-C14, and C16-C27 portions. The synthesis of the C3-C8 lactone used (R)-glycidol as the origin of the C4 stereogenic center and featured a late-stage optional incorporation of the C7 hydroxyl group. The complementary C9-C14 fragment was synthesized in a concise route from (R)-3-tert-butyldimethylsilyloxy-2-methylpropanal and propargyl bromide. Assembly of the C3-C14 spiroketal-containing intermediate from the constitutent fragments revealed a dramatic effect of C7 functionalization upon spiroketalization efficiency. In contrast, both (9E)- and (9Z)-enones converged readily to the C8 spiroketal upon treatment with acid. Modifications to the central C16-C27 fragment of okadaic acid included the early replacement of benzylic protecting groups by more suitable functionalities to facilitate both the generation of the C15-C27 intermediate and the deprotection of the final products. These modular building blocks were deployed for the synthesis of the C1-C27 scaffold of 7-deoxyokadaic acid. This work demonstrates improvements in the formation of versatile okadaic acid intermediates, as well as a reordering of fragment couplings. This alternative order of coupling was designed to promote the late stage incorporation of nonnatural lipophilic extensions from the C27 terminus.     

Model studies on a diastereoselective synthesis of the C(33)-C(37) fragment of Amphotericin B

A new, short and highly diastereoselective synthetic route aiming at the C(33)-C(37) fragment of Amphotericin B has been developed. Studies with a model aldehyde (benzaldehyde) have given very promising results: the desired stereochemistry of all four stereocenters of the target molecule has been achieved with high diastereoselection. The stereochemistry of three key intermediates and the target segment has been confirmed by X-ray crystallography.     

Palladium-catalyzed enantioselective domino heck-cyanation sequence: development and application to the total synthesis of esermethole and physostigmine

An efficient synthesis of functionalized 3-alkyl-3-cyanomethyl-2-oxindole 1 by a palladium-catalyzed domino Heck-cyanation reaction has been developed. Reaction of ortho-iodoanilide 5 with potassium ferro(II)cyanide, K(4)[Fe(CN)(6)], dissolved in DMF in the presence of palladium acetate and sodium carbonate afforded oxindole 1 in good to excellent yields. An enantioselective domino Heck-cyanation process has been developed for the first time using (S)-DIFLUORPHOS as a chiral supporting ligand, and an enantioselectivity of up to 79 % ee in the enantiomerically enriched oxindole was obtained under optimized conditions. A concise total synthesis of esermethole and physostigmine, powerful inhibitors of acetyl- and butyryl-cholinesterase, is documented.     

A Domino Approach to the Enantioselective Total Syntheses of Blennolide C and Gonytolide C

The first enantioselective total syntheses of the tetrahydroxanthenone (?)‐blennolide C (ent‐ 4 ) and related γ‐lactonyl chromanone (?)‐gonytolide C (ent‐ 3 ) are reported. Key to the syntheses is an enantioselective domino‐Wacker/carbonylation/methoxylation reaction to set up the stereocentre at C‐4a. Various chiral BOXAX ligands were investigated, including novel (S,S)‐iBu‐BOXAX, and allowed access to chromane 8 in an excellent enantioselectivity of 99 %. The second stereocentre at C‐4 was established employing a diastereoselective Sharpless dihydroxylation. An extensive survey of (DHQ)‐ and (DHQD)‐based ligands enabled the preparation of both the anti‐isomer 14 a and the syn‐isomer 14 b in very good to reasonable selectivities of 13.7:1 and 1:3.7, respectively. While 14 a was further converted to ent‐ 3 and ent‐ 4 , 14 b was elaborated to syn‐acid 25 and 2′‐epi‐gonytolide C 28 .     

A cross metathesis strategy for the synthesis of highly functionalized conjugated cyanodienes: synthesis of the C3-C17 framework of (−)-borrelidin

A cross metathesis strategy for the synthesis of highly functionalized conjugated cyanodienes was developed and was successfully applied in the synthesis of the C3-C17 framework of (−)-borrelidin.     

Efforts towards the synthesis of microsporin B: ready access to both the enantiomers of the key amino acid fragment

Both the isomers methyl-(2S,8R)-2-((tert-butoxycarbonyl)amino)-8-hydroxydecanoate and methyl-(2S,8S)-2-((tert-butoxycarbonyl)amino)-8-hydroxydecanoate of an unusual amino acid residue and the key fragment of microsporin B are prepared. The key steps include cross metathesis and enzymatic kinetic resolution. In addition, a linear tetrapeptide with desired components towards total synthesis is also reported.     

Total synthesis and evaluation of the actin-binding properties of microcarpalide and a focused library of analogues

A comparative investigation shows that hydroxylated 10-membered lactones modeled around the fungal metabolites microcarpalide (1) and pinolidoxin (2) are endowed with selective actin-binding properties. Although less potent than the marine natural product latrunculin A, which represents the standard in the field, nonenolides of this type are significantly less toxic and accommodate substantial structural editing. Most notable is the fact that even an intramolecular transesterification with formation of a hydroxylated butanolide skeleton does not annihilate their microfilament disrupting capacity. This finding calls for a reinvestigation of the biological profile of other fungal metabolites that embody a similar motif. Microcarpalide (1) serving as the calibration point for this comparative study was prepared by total synthesis based on ring-closing metathesis (RCM) as the key step. The chosen route favorably compares to previous approaches to this target and provides further support for the notion that the (E,Z)-configuration of a medium-sized cycloalkene can be controlled by proper choice of the catalyst as previously outlined by our group. 9-epi-Microcarpalide 26 and furanone 27 as representative examples of the "natural productlike" compounds investigated herein have been characterized by crystal structure analysis.     

Total synthesis of gambierol: the generation of the A-C and F-H subunits by using a C-glycoside centered strategy

Gambierol, a representative of the marine ladder toxin family, consists of eight ether rings, 18 stereocenters, and two challenging pyranyl rings having methyl groups that are in a 1,3-diaxial orientation to one another. Herein we describe the generation of gambierol's A-C and F-H ring systems and demonstrate the versatility of the glycosyl anhydride, enol ether-olefin RCM strategy to fused polycyclic ethers. This work has both enabled us to generate sufficient quantities of the gambierol precursors and has enabled us to better understand the chemical transformations that were key to these efforts. Fundamental work included efforts to C-glycosides and C-ketosides, Claisen rearrangements, and enol ether-olefin RCM reactions.     

Facile and efficient synthesis of lactols by a domino reaction of 2,3-epoxy alcohols with a hypervalent iodine(III) reagent and its application to the synthesis of lactones and the asymmetric synthesis of (+)-tanikolide

The domino reaction of 2,3-epoxy-1-alcohol derivatives, namely tetrasubstituted 2,3-epoxy-1-alcohols and 2- or 3-alkyl trisubstituted 2,3-epoxy-1-alcohols, with PhI(OCOCF(3))(2) in the presence of H(2)O is described in detail. In this reaction, several types of lactol derivatives can be directly obtained from the 2,3-epoxy-1-alcohol derivatives in a single operation. The obtained lactols were successively converted into the corresponding lactones. This reaction is applicable to the construction of optically active lactone compounds. The asymmetric total synthesis of (+)-tanikolide, an antifungal marine natural product, has been effectively achieved by using this reaction.